Me present invention relates to broadband communications and digital subscriber line (DSL) technologies. More particularly, the present invention relates to amplifiers and line drivers employing active termination to synthesize the output impedance.
FIG. 1 illustrates a traditional line interface 10 including a line driver amplifier 12, a hybrid circuit 14, and a transformer 16. The transformer 16 has a turns ratio of 1:n, and couples a transmit signal (TX) from the line driver amplifier 12 to the transmission line 18 having a load impedance Z (typically Z=100xcexa9). A matching resistor (often referred to as a back termination resistor Rbt) is usually required between the amplifier 12 and the transformer 16 to implement a full duplex transmission and hybrid rejection. The value of the back termination resistor Rbt is selected to match the line impedance RL seen by the amplifier 12, and each of the back termination resistors Rbt has a value of RL/2 for a differential structure as shown in FIG. 1.
Although the back termination resistors Rub are necessary in order to properly terminate the receive signal and also to detect receive signal developed across the resistors, they waste one half of the power provided by the line driver amplifier 12. Therefore, the signal swing VA output from the amplifier 12 is twice as large as the signal swing VB supplied to the transformer 16. That is, the same amount of power as that is required to the transmission line 18 is dissipated at the matching resistors Rbt.
The purpose of active termination or synthesizing impedance is to reduce the power dissipation of the amplifier in the back termination resistors. FIG. 2 illustrates one scheme 20 to implement active termination to synthesize the output impedance of the amplifier, which simulates the back termination resistor within the amplifier itself. As shown in FIG. 2, an output amplifier 22 has two output stages 24 and 26, and an internal resistor 28 (having a resistance Rint) coupled between the two stages. The first output stage 24 includes a first output transistor, such as a metal oxide semiconductor field effect transistor (MOSFEI) M1, and is coupled to a node 23. Similarly, the second output stage 26 includes a second output transistor M2, and is coupled to an output node 25. The device ratio of the first transistor M1 and the second transistor M2 is 1:N.
The output impedance Zout is given by             Z      out        =                  R        int                    1        +        N              ,
and the resistance Rint is determined so that the output impedace Zout matches the line impedace RL seen by the amplifier 22. Since the internal resistor 28 is provided within the two output stages 24 and 26 of the amplifier 22, there is no matching resistor between the output node 25 of the amplifier 22 and the transformer. Therefore, the signal swing of the amplifier output (i.e., at the node 25) is directly supplied to the transformer, and thus is reduced by half compared to the conventional structure (FIG. 1) as described above, thereby reducing the required power of the amplifier 22.
A major drawback of the active termination structure shown in FIG. 2 however is that the synthesized impedace has sensitivity to line impedance variations, because the second output transistor 26 is not inside the closed loop configuration, as shown in FIG. 2. In the conventional active termination scheme 20, the line impedance Z is assumed to be constant and thus the synthesized output impedance Zout has a fixed value so as to match the constant line impedance Z. However, the actual line impedance is not constant and varies with frequency, and the frequency dependency of a transmission line also varies with the type of the transmission line, resulting in a mismatch of the synthesized impedance. Such a mismatch between the synthesized impedance and the actual line impedance degrades the linearity of the amplifier.
Typical achieved linearity level using the traditional active termination structure is around 40 dB. This level of linearity may be sufficient for voice-band communications, however, it is not acceptable in broadband communication applications such as xDSL transceivers. There is no known approach to solve this problem employing full active termination. Accordingly, it would be desirable to provide means for compensate such line impedance variation so as to improve the linearity of an amplifier and a line driver used for broadband communications.
A line driver for coupling a data Deceiver to a transmission line having a load impedance via a transformer with a turns ratio of 1:n includes input port for receiving an input signal voltage from the data transceiver, an output port for supplying an output signal voltage to the transformer, and an amplifier circuit coupled with the input port, for amplifying the input signal voltage. The amplifier circuit includes a first output stage, a second output stage coupled to the output port, an output resistor coupled to the first output stage, a feedback path from the first output stage to an input of the amplifier circuit, and a line matching network coupled between the first output stage and the second output stage, for compensating variations in the load impedance, so that a synthesized output impedance of the line driver substantially matches an actual load impedance Z of the transmission line.